This application is entitled to the benefit of and incorporates by reference in their entireties essential subject matter disclosed in International Application No. PCT/EP00/02560 filed on Mar. 23, 2000, Luxembourg Patent Application No. 90 376 filed on Mar. 23, 1999 and Luxembourg Patent Application No. 90 475 filed on Nov. 19, 1999.
The present invention generally relates to the manufacturing of a multilayer printed circuit board and to a composite foil for use therein.
The development of very compact and powerful electronic devices has been possible thanks to high-density printed circuit boards (PCB), obtained by sequential build-up (SBU) technology. Basically, a build-up multilayer circuit is a combination of several superimposed layers of different wiring densities, which are separated by dielectric layers and interconnected through micro blind vias with diameters of generally less than 100 xcexcm.
Nowadays, essentially three different technologies are available for the manufacture of microvias: (1) the photochemical etching of photodielectrics; (2) the plasma etching process; and (3) the still relatively new process of laser drilling. Laser drilling seems to be the most promising technology for the production of microvias. Excimer, Nd-YAG and CO2 laser sources are currently used for drilling of microvias, but each of these laser sources still has its specific draw-backs. Excimer lasers are not considered economically viable for industrial use. They have a low ablation rate per pulse and involve high investments in safety precautions, as excimer laser gases are extremely corrosive and highly toxic. Nd-YAG lasers are successfully used for smaller and medium sized volumes of high end products with microvias of diameters from 25 to about 75 xcexcm. Larger holes must be produced by trepanning (i.e. by drilling multiple smaller holes), which of course reduces drilling speeds considerably. CO2 lasers are increasingly gaining ground vis-à-vis the Nd-YAG laser for a large volume production of microvias. They are characterised by an ablation rate in non-reinforced polymer that is about twenty times as high as for Excimer or Nd-YAG lasers.
However, if CO2 lasers are very much adapted for polymer ablation, they are not suitable for copper removal. Hence, an additional process step, the manufacturing of a conformal mask, is necessary before a hole can be produced in the dielectric layer with the CO2 laser. During this additional step, openings are etched in the copper laminate at the positions where the dielectric should be removed later. This method allows to use the CO2 laser for drilling blind microvias, but the manufacturing process is slowed by the conformal mask building step and there is a real risk of damaging the copper layer during the conformal mask building.
In order to avoid the above and other disadvantages of the conformal mask technology, it has been suggested to use a twin laser device for drilling the holes. Such a twin laser device is a combination of CO2 laser source with an IR solid-state laser. First, the opening in the copper foil is carried out with the solid-state laser. The resin layer is then removed with the CO2 laser. Such a twin laser allows microvia drilling in copper cladded build-ups, but the investment cost is higher than for a simple CO2 laser, and the slow copper drilling step is responsible for a low process speed.
It has also been suggested to replace the manufacture of the conformal mask by a xe2x80x9chalf etchingxe2x80x9d step. A thin resin coated copper foil of about 18 xcexcm is first laminated on the core board, with its copper foil upside. After lamination, the 18 xcexcm copper foil is etched over its entire surface, in order to reduce its thickness down to about 5 xcexcm. In the next step, the copper layer undergoes a black oxide treatment, to form a laser drilling adapted surface. Then, the CO2 laser is used to drill the microvias directly through the 5 xcexcm copper layer and the subjacent resin layer. The xe2x80x9chalf etchingxe2x80x9d step is of course less complex than conformal mask building, but the manufacturing process is nevertheless slowed down by the half etching step and the copper surface might still be damaged during the half etching step. Furthermore, CO2 laser drilling on xe2x80x9chalf etchedxe2x80x9d copper foils does not yet produce satisfying results. The poor results are due to the fact, that etching the entire surface of e.g. a 600 mmxc3x97500 mm printed circuit board is neither a homogeneous, nor a precise operation. The most recent etching agents and etching machines claim a tolerance of xc2x12 xcexcm. The thickness of a copper foil etched down to a nominal thickness of 5 xcexcm may therefor vary from 3 xcexcm to 7 xcexcm. When drilling the microvias, the laser energy is set for a nominal copper thickness of 5 xcexcm. If the copper layer at the incidence point is only 3 xcexcm, the set laser energy is too high for the amount of copper to be vaporised. As a result, copper splashes are created on the border of the hole and the hole in the dielectric material is generally misshaped. If the copper layer at the incidence point is however 7 xcexcm, the set laser energy is too low and the resulting hole in the dielectric material will have too small a diameter or will even not extend to the subjacent copper layer. Due to the disappointing results of the half etching method, CO2 laser drilling is still exclusively used on non-copper cladded build-up materials or with conformal mask etching.
U.S. Pat. No. 3,998,601 discloses a composite foil and a method for manufacturing the latter. The composite foil comprises an electrodeposited copper support layer and a second electrodeposited copper layer of a thickness which is-not self supporting. Intermediate the copper support layer and the second copper layer is a thin layer of a release agent, preferably chromium. The second copper layer has a thickness no greater than 12 xcexcm. A laminate may be formed by superimposing this composite foil on epoxy impregnated fiberglass with the ultrathin copper surface in contact with the epoxy-glass substrate, and subjecting this assembly to a conventional laminating process. After cooling of the laminate, the copper carrier coated with the release agent is peeled away to produce a thin copper clad laminate suitable for etching, etc. in the production of printed circuit elements.
A method for manufacturing a multi-layer interconnected board is described in JP 10 190236. According to a first step of this method, a circuit board with a desired circuit pattern formed thereon, a metal foil and an insulator layer are positioned, stacked up and laminated. In the next step, a point on a conductor layer desired to be laser processed is subjected to a process to increase the rate of absorption of the laser. In the following step a laser beam is impinged on the processed point so as to melt and sublime the metal foil and the insulator layer and thereby form a hole. In a final step, electroless plating is performed to electrically connect conductors through the hole.
The possibility of laser drilling into copper clad epoxy-glass, in particular by means of a CO2 laser, is reported in xe2x80x9cLaser drilling of microvias in epoxy-glass printed circuit boardsxe2x80x9d by A. Kestenbaum et al., IEEE Transactions on components, hybrids and manufacturing technology, vol.13, no. 4, Dec. 1990 (1990-12), pages 1055-1062, XP000176849 IEEE Inc. New York, US ISSN: 0148-6411. In one of the experiments, a CO2 laser was used to drill a through hole in a 0.254 mm (10-mil) epoxy-glass layer clad with 4.4 xcexcm (xe2x85x9-oz) copper on both sides. In another experiment a CO2 laser was used to drill a blind hole in a 0.254 mm (10-mil) epoxy-glass layer clad with 4.4 xcexcm (xe2x85x9-oz) copper.
DE-A-31 03 986 relates to a process for the production of drilled holes for the throughplating in printed circuit boards consisting of substrate materials on the basis of carbon. The throughholes are drilled using a CO2 laser. The metal layer on top of the printed circuit board may be coated with a radiation-specific acceptor to improve the absorption of the laser beam. In case the metal layer is made of copper, the acceptor may be made of Copper-II-oxide.
Consequently, there is a strong need for a simple and efficient method for the manufacture of multilayer printed circuit boards, which allows fast laser drilling of high-quality microvias.
Another object of the present invention is to provide a composite foil, which allows fast laser drilling of high-quality microvias, when it is used in the manufacture of multilayer printed circuit boards.
In accordance with the present invention, a method for manufacturing a multilayer printed circuit board comprises the following steps:
a) providing a core board;
b) providing a composite foil including a functional copper foil of less than 10 xcexcm mounted on a carrier foil, said copper foil having a front side facing said carrier foil and a back side coated with a non-reinforced thermosetting resin;
c) laminating said composite foil with the resin coated back side on one side of said core board;
d) removing said carrier foil from said functional copper foil, in order to uncover said front side of said functional copper foil;
e) drilling holes through said functional copper foil and said resin in order to form microvias.
According to an important aspect of the present invention, the functional copper foil of the composite foil has a thickness of less than 10 xcexcm, preferably of about 5 xcexcm, whereby it becomes possible to use a CO2 laser source to drill microvias directly from the uncovered front side through the very thin functional copper foil and the subjacent dielectric layer. It follows that xe2x80x9chalf etchingxe2x80x9d or xe2x80x9cconformal mask buildingxe2x80x9d steps are no longer necessary, so that the manufacturing process of a multilayer PCB gets simpler. The simplicity of the process enables high speed processing and high productivity, with less process equipment and therefore lower investment costs. In other words, the process of manufacturing gets more efficient. Consumption of chemical etching agents is also substantially reduced. This is of course an important feature with regard to environmental protection. With regard to quality control, it will be noted that the thin functional copper foil has an accurate thickness and a controlled and homogenous surface profile and roughness, so that the CO2 laser beam encounters similar and reproducible drilling conditions everywhere. It follows that the laser energy can be set to drill very precise microvias everywhere on the PCB, i.e. microvias having a well determined shape, diameter and height, without producing copper splashes on the copper surface. It will further be appreciated that the carrier provides the necessary rigidity for handling the functional resin coated copper foil. Moreover, the latter is protected between its carrier and its resin coating against particles, chemical agents or atmospheric agents, that may damage the surface integrity, and alter the future circuit pattern. Due to the self supporting carrier foil, not only the very thin functional copper foil, but also the rather brittle resin coating is protected against tears, cracks and wrinkles. During lamination, the carrier provides an efficient protection of the very thin functional copper foil against dust and particles (as e.g. resin particles), which may indent the surface, and against resin bleed-through. After removal of the carrier, the functional copper layer is consequently clean and free of any defects such as e.g. indentations, tears, cracks and wrinkles.
The functional copper foil is preferably obtained by electro-deposition. Advantageously, the front side of the functional copper foil has received a surface preparation favouring the absorption of CO2 laser light. Such a surface preparation may e.g. provide a front side having a particular surface profile and roughness and/or a colour favouring the absorption of CO2 laser light. It can take place during manufacturing of the composite copper foil, so that the functional copper foil is ready for laser drilling after removal of its carrier. The front side of the functional copper foil may also be covered prior to laser drilling with a black oxide conversion coating, thus favouring the absorption of CO2 laser light.
It will be noted that the composite foil preferably includes a release layer intermediate the carrier foil and the functional copper foil. Such a release layer may simply permit the separation of the carrier foil, like e.g. a thin, chromium based release layer. In this case, the, carrier removal then normally consists in mechanically peeling off the carrier foil and the release layer simultaneously, i.e. the release layer remains bonded to the carrier foil. However, another kind of release layer may remain on the functional copper foil instead of the carrier foil when removing the carrier foil, and exhibit a particular surface colour favouring the absorption of CO2 laser light. Such a kind of release layer, having a dual function, may be a dark coloured conductive material layer and should allow copper electroplating to form the functional copper foil thereon, show a strong adhesion to the functional copper foil, and have a colour favouring the absorption of the infrared light of a CO2 laser.
In a first embodiment, the resin is a B-staged resin. It can therefore adapt to the subjacent circuits of the core board, and the polymerisation is completed during lamination.
In a second embodiment, the resin coating on the back side consists of a C-staged resin layer applied to the back side of the functional copper foil, and of a B-staged resin layer applied to said C-staged resin layer. The insulating layer is therefore thicker and can still adapt to the subjacent circuit layer.
It will be appreciated that the present invention also provides a composite foil for use in a method for manufacturing a multilayer printed circuit board, comprising a self-supporting carrier foil, preferably a copper foil with a thickness from 18 to 150 xcexcm; a release layer on one side of the carrier foil; a functional copper foil, having a thickness of less than 10 xcexcm, most preferably of about 5 xcexcm, the functional copper foil being deposited on the release layer and having a front side facing the release layer and a back side; and a nonreinforced thermosetting resin coating on the back side of the functional copper foil.
The front side of the functional copper foil has preferably received a surface preparation favouring the absorption of CO2 laser light. Such a surface preparation may be carried out by forming a dark coloured conductive material layer between the release layer and the functional copper foil. In a first embodiment of the composite copper foil of the invention, the dark coloured conductive material layer may comprise carbon black and/or graphite. In a second embodiment, the dark coloured conductive material layer may comprise a dark coloured electrically conductive polymer layer.
It shall be noted that the release layer may itself be a dark coloured conductive material layer, thereby exhibiting a dual function of release layer and surface preparation favouring the absorption of CO2 laser light. The composite foil would then comprise a carrier foil, this release layer having a dual function, a functional copper foil, and a resin coating. It is clear that such a release layer, contrary to a conventional release layer like e.g. a chromium release layer, has to adhere to the front side of the functional copper foil when removing the carrier foil.
Advantageously, the back side of the functional copper foil has a bonding layer thereon so as to improve its bond strength with the resin coating. Moreover, the functional copper foil may be covered with a passivation layer, preferably intermediate the bonding layer and the resin coating, in order to warrant the stability of the, back side.